In thin-wall synthetic resin containers whose body portion and bottom portion are bi-axially oriented by orientation blow molding, the neck portion of the container which is formed by preform injection or extrusion molding is in a non-oriented state and exhibits poor heat resistance characteristics. Therefore, in containers such as bottles formed of polyethylene terephthalate resin, the neck portion is often deformed after the container is heated.
In view of the foregoing, as shown in FIGS. 5 and 6, there has been contemplated a neck portion 20 having a three-layer construction which is resistant to heat. The neck portion 20 includes a heat resistant layer 21, formed of a thermoplastic resin excellent in heat resistance, which is surrounded by an inner layer 22 and an outer layer 23 formed of a neck-forming resin.
In the above-described conventional construction, the heat resistant layer 21 prevents the peripheral portion of the neck portion 20 from being deformed as a result of the application of heat. However, the open end 24 of the neck portion is still subject to heat deformation. More particularly, the outer peripheral edge 25 of the outer layer 23 produces a partial strain due to the heat deformation of the open end 24, thus posing the problem that the contents of the container leak after a cap has been sealed.